1. Field of the Invention
The present invention relates to a color registration correction system for an electro-photographic printer. More particularly, the present invention relates to an apparatus for and a method of correcting color registration in an electro-photographic printer, by which correction values of the color registration can be determined without errors.
2. Description of the Related Art
A typical electro-photographic printer such as a color laser printer includes four photosensitive drums, an exposure unit, a developing unit, and an image carrying medium, such as a transfer belt. Four photosensitive drums correspond to four colors, such as yellow, cyan, magenta, and black, respectively. The exposure unit projects light beams to each photosensitive drum to form electrostatic latent images which corresponds to a desired image.
The developing unit develops the electrostatic latent images formed by the exposure unit for each color with a developing solution. The image carrying medium receives the images developed in the photosensitive drums in turn, and the received images are superimposed to form a complete image. Then, the complete image is transferred to a sheet of paper.
In order to print a clear color image, it is important to accurately arrange start positions for each color of the developed images to be transferred from each photosensitive drum. It is also important to accurately arrange end positions where the transfer of the images is complete. For this purpose, exposure start points in the exposure unit should be accurately set up for each photosensitive drum based on a driving speed of the image carrying medium. The accurate setting of the exposure start points for each color is referred to as color registration.
However, even when the exposure start points are accurately set up to initial values, registration errors may be generated. For example, when printing may be effected by expansion of a roller for driving the image carrying medium may be expanded due to the heat generated during the printing. In other words, if the diameter of the roller is changed, the transfer speed of the image carrying medium is correspondingly changed even when the rotation rate of the roller is constant. Therefore, proper color registration cannot be achieved even when the exposure is started at the exposure start points based on the initial values. Consequently, the exposure start points set to initial values should be dynamically controllable and correctable based on information from the surrounding conditions to obtain a clear color image. Such dynamic correction of the registration errors is referred to as color registration correction.
In conventional color registration correction, the exposure unit forms particular marks on each photosensitive drum, and the marks are developed and transferred to the image carrying medium. Then, positions of the marks are detected by sensors to identify registration errors in positions of the marks for each color. Correction values are calculated based on the detected positions of the marks to rearrange the exposure start points for each color. In this case, if there is any defect or disturbance around the marks on the surface of the image carrying medium, the sensors may detect the defect as a true position mark, and a false correction value may be calculated.